1. Field of the Invention
The present invention relates generally to telemetry apparatuses and methods, and more particularly to acoustic telemetry relay network timing for exploration, completion and production wells for hydrocarbons and other resources, and for other telemetry applications.
2. Description of the Related Art
Acoustic telemetry is a method of communication used in the well drilling, completion and production industries. In a typical drilling environment, acoustic extensional carrier waves from an acoustic telemetry device are modulated in order to carry information via the drillpipe as the transmission medium to the surface. Upon arrival at the surface, the waves are detected, decoded and displayed in order that drillers, geologists and others helping steer or control the well are provided with drilling and formation data. In production wells, downhole information can similarly be transmitted via the well casings. Acoustic telemetry transmits data to the surface in real-time and is independent of fluid flow, depth, well trajectory and other drilling parameters.
The theory of acoustic telemetry as applied to communication along drillstrings and well casings has a long history, and a comprehensive theoretical understanding has generally been backed up by accurate measurements. It is now generally recognized that the nearly regular periodic structure of drillpipe and well casings imposes a passband/stopband structure on the frequency response, similar to that of a comb filter. Dispersion, phase non-linearity and frequency-dependent attenuation make drillpipe a challenging medium for telemetry, the situation being made even more challenging by the significant surface and downhole noise generally experienced.
When exploring for oil or gas, in coal mine drilling and in other drilling applications, an acoustic transmitter is preferentially placed near the BHA, typically near the drill bit where the transmitter can gather certain drilling and geological formation data, process this data, and then convert the data into a signal to be transmitted up-hole to an appropriate receiving and decoding node. In some systems, the transmitter is designed to produce elastic extensional stress waves that propagate through the drillstring to the surface, where the waves are detected by sensors, such as accelerometers, attached to the drill string or associated drilling rig equipment. These waves carry information of value to the drillers and others who are responsible for steering the well. Examples of such systems and their components are shown in: Drumheller U.S. Pat. No. 5,128,901 for Acoustic Data Transmission through a Drillstring; Drumheller U.S. Pat. No. 6,791,470 for Reducing Injection Loss in Drill Strings; Camwell et al. U.S. Pat. No. 7,928,861 for Telemetry Wave Detection Apparatus and Method; and Camwell et al. U.S. Pat. No. 8,115,651 for Drill String Telemetry Methods and Apparatus. These patents are incorporated herein by reference.
Acoustic communication through drilling and production strings (collectively “drillstrings”) is generally limited by available frequency spectra and signal attenuation. Consequently, transmission data rates tend to be relatively low, e.g., in the range of tens of bits per second, and multiple repeater nodes have previously been used to boost the telemetry signals and overcome the problem of acoustic signal attenuation and associated range limitations. The inclusion of multiple acoustic transceiver nodes within a drillstring forms a low rate linear repeater data network. As used herein “nodes” are defined as receivers (Rx), transmitters or transceivers (Tx) for telemetry signals traveling between adjacent pairs of nodes. Alternatively, the nodes could be associated with and referred to as “stations” (e.g., ST0, ST1, . . . STn) located along the drillstring. The low data rate linear repeater networks suffer from high latency (time for data to propagate through the network) due to the time it takes for each node to receive data packets and relay data onward. An objective of repeater networks is to relay data as quickly as possible after initial receipt, in order to minimize latency of data delivered to the surface (or other destination) and to maximize data throughput.
The latency of delivered measurement data translates into a potentially large time difference between the time at which a downhole sensor measurement is made and when that value is delivered to the surface, obscuring potentially valuable correlation between downhole and uphole events. Additionally, as sensor acquisition at each node within the network occurs at different points in time, the accuracy of inter-node differential measurements is limited, impairing the ability to discern transient events traversing the string.
A possible solution to drillstring acoustic communication latency-associated problems is to include time-of-measurement information with transmitted information from each node. However, bandwidth limitations make the inclusion of time-of-measurement (e.g., sensor acquisition time) information overhead in the acoustic packets undesirable, and require all downhole clocks to be very accurately aligned, which can be problematic given the significant temperature differentials across the networks (e.g., 150° C. or more) and the long periods of continuous network operation.